The present invention is directed to high-speed printers and, more specifically, to the control of a printing ribbon in a moving font printer.
For many years it was common for virtually all typing and printing equipment to include a type font or group of letter impact members designed to make an impact at a fixed location in reference to the main carriage of the machine. As more and more sophisticated electronics were integrated with such printing equipment, however, the difficulty in moving a platen and its associated carriage at high speed suggested the use of a moving font machine, that is, a machine where the paper remains stationary as a line is typed, while the type font or impact mechanism strikes at different locations along the platen by moving from one side of the main machine frame toward the other. This type font and its associated carriage are considerably lighter than the platen and its associated carriage assembly, and thus more responsive to lightweight servo systems (or mechanical systems) used to advance its position.
In most applications of this type, the font carriage mechanism supports supply and take-up reels for the printing ribbon so that the ribbon can be incremented relative to the printing head without reference to the position of the printing head relative to the platen. Such a system permitted extremely high quality carbon ribbon to be used, since it eliminated the need for overlapping impacting of characters on the ribbon. In addition, with a system of this type where the ribbon need only span the width of the type font carriage, the ribbon can be made relatively narrow, so that substantially all of the ribbon surface could be utilized in a single printing impact pass.
While it is, of course, advantageous to reduce the weight of the moving type font carriage to the greatest extent possible, the prior art has not provided a satisfactory means for removing the printing ribbon, supply reel and take-up reel, and associated drive motors from the type font carriage. This limitation in the prior art appears to result from a number of factors. Initially, if the ribbon is to be mounted on the machine's main frame, it must span the width of the machine so that the moving type font can index along the length of the ribbon as it progresses from one end of a typing line to the other. This requires that the supply reel be mounted on one extreme end of the typing frame and the take-up reel at the other extreme end. The ribbon stretched between these reels must be relatively strong in order to maintain its stretched position, since intermediate guides interface with the printing operation. This requires that either the thickness or the width of the ribbon be increased. Increases in the ribbon thickness reduce the printing quality and, therefore, the only apparent solution was an increase in ribbon width in order to make the ribbon strong enough to serve this purpose. Unfortunately, this increase in ribbon width, when coupled with the desire to avoid letter overstrikes on the ribbon, required that most of the ribbon's usable surface be wasted, a single pass utilizing the center portion of the ribbon exclusively.
An additional problem associated with any attempt to mount the ribbon reels on the machine's main frame is the fact that, after the printer has printed an entire line of type and the carriage is returned, assuming that the printer does not print in both directions, the ribbon motors must have, in effect, a fly back system. Such a system would move the ribbon very rapidly during carriage return to place an unused length of ribbon in front of the printer platen. Alternatively, if the printer is to operate in both a forward and reverse direction, one direction following the other, it is still necessary to have a ribbon fly-back. The fly-back in this instance must limit the speed of the machine substantially, since, after printing in one direction if overstrikes on the ribbon are to be avoided, the printing process must pause while the ribbon flies back, so that a fresh length of ribbon is present for the reverse printing operation.
As a consequence of these difficulties, the common technique in the prior art has been to mount the supply and take-up reels for the ribbon on the printing font carriage, increasing the size and weight of the servo system a sufficient extent to drive the carriage with this additional weight at the required speed. Such increases in the size of the servo (or the mechanical system in a mechanical machine) of necessity increases the cost, reduces the reliability and limit to a practical extent the speed at which the machine can operate.